Cyclically operated or oscillating valves for regulating the flow of a fluid or gaseous medium are used in many applications. To ensure proper operation of a device or a process, it is desirable to monitor the mechanical function of such valves. By monitoring the valve or valves, it is possible to limit or prevent the occurrence of breakdowns and/or emissions caused by valve failure.
In general, vehicles are provided with a purge system to prevent fuel evaporated in a fuel tank from being discharged into the atmosphere. Instead, the evaporated fuel is absorbed in a canister containing activated carbon, which canister is placed in a conduit connecting the fuel tank and the intake pipe of the engine. The fuel absorbed by the canister over a period of time is released to the engine by a controllable purge valve. When the purge valve is opened, ambient air flows through the canister and draws fuel vapor into the engine. The direction of flow and the flow rate is a function of the pressure difference between the atmospheric pressure and the engine intake pressure. The purge valve is arranged to open only when the pressure differential between the atmosphere and the intake pipe is sufficient to cause flow in a desired direction.
A malfunction of the purge valve may cause increased fuel consumption, increased tailpipe emissions, and increased fuel escaping from the tank or the canister.
U.S. Pat. No. 5,780,728 discloses an arrangement provided with a pressure sensor in a purge line. The sensor is adapted to measure both the pressure in the purge line and in the engine intake pipe. The purge valve can be controlled in relation to the pressures and a number of further conditions, such as engine load, throttle opening and fuel injection pulse duration. By using a number of available signals and by adapting an existing pressure sensor for measuring purge line pressure, the system can be diagnosed without introducing further sensors. However, additional conduits and switching gear must be installed to connect the pressure sensor to both the purge line and the intake pipe. The function of the purge valve cannot be directly monitored.
U.S. Pat. No. 6,082,337 discloses an arrangement for diagnosing a purge system that has pressure sensors both in the fuel tank and the intake pipe. However, the arrangement is mainly directed toward monitoring leakage. The system can control an electromagnetic purge valve, but does not monitor its mechanical function.
U.S. Pat. No. 6,131,448 discloses an arrangement that diagnoses a purge system by estimating the space volume of the system using two duty ratios for the purge valve. The result can be used for detecting leakage in the system, but is not suitable for monitoring the purge valve function.
None of the known diagnostic arrangements disclose a method or an arrangement for monitoring the function of or performing diagnostic tests on a valve, such as a purge valve. This is required to ensure proper function and so that a warning is transmitted to the control system if a malfunction should occur. Hence, there exists a need for a simple and inexpensive solution to the problem of diagnosing the mechanical function of oscillating valves or other types of controllable valves for controlling a gaseous or fluid flow between two volumes, such as a purge valve for controlling the flow of fuel vapor from a canister to an engine intake pipe.